Advanced combustion processes are being developed in the engine combustion arts to reduce emissions of NOx and PM from both spark-ignited (SI) and compression-ignited (CI) engines, especially light-duty automotive engines. As used herein, “spark-ignited” and “spark ignition” refers to any internal combustion engine wherein ignition of a compressed combustible mixture of fuel and air in an engine cylinder occurs principally because of an electric discharge formed in the midst of the compressed combustible mixture. “Compression-ignited” and “compression ignition” refers to any internal combustion engine wherein ignition of a compressed combustible mixture of fuel and air occurs principally because some or all of the components of the compressed combustible mixture have been adiabatically compressed in a cylinder to a temperature at or above the spontaneous ignition temperature of the mixture. Thus, as used herein, “compression-ignited” and “compression ignition” should be taken to mean not only conventional prior art diesel ignition, wherein fuel is injected into a compressed air charge at substantially the top of the compression stroke to form a non-homogeneous mixture, but also all other compression-type ignition including but not limited to homogeneous charge compression ignition (HCCI), controlled autoignition (CAI), and premixed diesel (PMD) ignition.
HCCI-diesel is homogeneous charge CI using diesel fuel. It is mixed mode in that this engine must revert to conventional diesel combustion in some conditions. Fuel may be injected early to foster mixing. HCCI-diesel is controlled using VVA and other means such that the charge is diluted and combustion temperatures are low. This produces combustion with low soot and low NOx.
HCCI-gasoline is also homogeneous charge compression ignition but it uses gasoline. For certain conditions such as idle and higher loads, it reverts to conventional spark ignition. The combustion process is similar to HCCI-diesel but a lower pressure, and a lower cost injection system is utilized.
HCCI, whether HCCI-diesel or HCCI-gasoline, is known to be chemically-kinetically controlled. These combustion processes require special in-cylinder conditions, operate in critical ranges, and generally are difficult to control. If successful, however, HCCI promises drastically reduced emissions that may satisfy future US and European emissions standards. Avoidance of high cost and complexity is a significant challenge for many advanced engine concepts.
Advanced engines are foreseen and in development in the engine arts which may use diesel fuel, gasoline, mixtures thereof, or other specialty fuels. Fuel may be port-injected and/or cylinder-injected to foster homogeneous charge compression ignition (HCCI) or “controlled autoignition” (CAI) to provide controlled, low-temperature burning of the fuel. Hybrid engines may, for example, utilize homogeneous charge compression ignition (HCCI) in some operating modes and spark-ignition (SI) or conventional compression-ignition (CI) in other modes. Thus, the scope of the present invention applies to both conventional combustion modes and advanced premixed combustion modes for gasoline-fueled and diesel-fueled engines. A main need in the art is a simple solution to satisfy HCCI requirements while reducing cost and complexity of the overall powertrain.
Because combustion initiation is chemically-kinetically controlled, one problem of HCCI systems is that parameters other than fuel injection timing (as for conventional diesel engines) must be controlled in order to control combustion initiation. Mixture compression temperature and exhaust gas recirculation (EGR) level are two such control parameters. The Miller Cycle, known in the prior art, with variable late intake valve closing (LIVC) and turbo compounding can be used to control compression temperature over the engine operating range. The Miller Cycle with LIVC provides independent control of compression ratio (CR) and expansion ratio (ER), with CR generally lower than ER.
Currently, light-duty diesel engines operate with fixed geometric compression ratios of about 18:1 to about 22:1 in order to achieve good cold-starting characteristics. However, once warmed up, such compression ratios are excessively high and reduce thermodynamic efficiency due to high heat losses. Such high compression ratios contribute to higher peak cycle temperatures, which exacerbates NOx production. As with other internal combustion engines, the diesel engine operates within a peak cylinder pressure constraint that is dictated by structural strength considerations. In other words, higher CR demands a more massive, heavy, and expensive engine than lower CR.
Thus, a further need in the CI art is an engine having a lower CR when warmed to permit lower peak cylinder temperatures and pressures.
Variable LIVC is useful to provide high effective compression ratio (ECR) for good cold start characteristics, while providing lower ECR and lower compression temperatures for warmed-up operation. In some applications, the use of variable LIVC can eliminate the need for prior art glow plugs (GP) for engine starting. GP elimination has at least one important side benefit in that real estate is freed up on the engine head for a flush-mounted cylinder pressure transducer. Cylinder-pressure-based control may be necessary for optimal HCCI systems and is currently under study in the engine arts.
As noted above, EGR is an important control parameter for HCCI. Generally, HCCI systems require relatively high levels of exhaust gas recirculation, as high as 50% to 70%, and the combustion process is sensitive to small changes in EGR level. EGR can be used to control both combustion initiation and combustion burn rates, while also lowering flame temperatures for reduced NOx emissions. Prior art external EGR systems, wherein exhaust gas is metered from the exhaust manifold into the intake manifold, offer the advantage of cooling the exhaust gas for reduced NOx, but these systems are bulky, expensive, and slow to respond.
Thus, a still further need in the art is means for rapid control of EGR introduction into the firing chamber for good transient response of advanced HCCI systems.
Another requirement of HCCI systems is in-cylinder swirl of intake gases to provide effective mixing of injected fuel and air. Swirl can be produced in the prior art by swirl ports, but such devices limit full-load airflow and engine power. Alternatively, port deactivation (PDA) can provide swirl by blocking one intake port of a two-intake-valve system with a butterfly valve, barrel valve, or slider valve. PDA systems are used widely in the prior art but they suffer from deposit accumulation downstream of the PDA device and may introduce a flow loss due to shafts or other mechanisms blocking airflow. Valve deactivation (VDA) is a preferred alternative method that involves deactivating one of the two intake valves. This method is advantageous because it avoids possible deposit problems and airflow restrictions.
Thus, a still further need in the art is a simple, inexpensive mechanism for providing valve deactivation of intake valves.
A desirable feature for advanced HCCI combustion systems is the ability to rapidly heat both the combustion chamber and the exhaust gas catalyst(s) during a cold start. Rapid heating of the combustion chamber walls can improve combustion within the first few seconds of operation during which the catalyst is inactive. Heating the catalyst more quickly shortens the time to catalyst light off and thereby shortens the period of uncatalyzed emissions.
One known method for heating the cylinder during a cold start is early exhaust valve closing (EEVC) through which hot burned gases are trapped in the cylinder just prior to fuel injection. This can be combined with increased effective compression ratio (ECR) by closing the intake valve near bottom dead center (early intake valve closing (EIVC)) for increased compression temperatures and good cold start characteristics.
One known method to accelerate heating the exhaust catalyst is early exhaust valve opening (EEVO), which effectively blows down the hot cylinder gases before expansion is complete. This can be combined with late combustion phasing for additional exhaust temperature increases.
Thus, a still further need in the art is a simple means for EEVC and EEVO.
Another problem with CI combustion in general, and HCCI combustion in particular, is the extremely low exhaust temperatures that are typically encountered for warmed-up conditions. Exhaust temperatures may drop below temperatures at which the catalyst is active, for example, while the engine is idling or at partial load. Exhaust temperatures below 150° C. are known to limit catalyst conversion efficiency.
As is known in the prior art, exhaust temperatures can be increased without a loss of engine efficiency by reducing trapped air mass in the cylinder. This enables lower air-fuel ratios at any fueling level and increases exhaust temperature. Some CI engines incorporate a throttle to reduce trapped air mass but throttling reduces engine efficiency. Alternatively, control of trapped air mass is possible by variable intake valve closing and variable charging using a variable nozzle turbocharger (VNT). Another method to increase exhaust temperature is cylinder deactivation (CDA), which effectively increases load factors in the remaining firing cylinders. Both approaches can be achieved by variable valve actuation mechanisms.
Thus, a still further need in the art is a simple means for providing variable valve actuation in a CI or HCCI engine.
Finally, while current diesel engines exhibit generally good low-speed torque compared to modern SI engines, greater levels of low-speed torque are highly valued in the industry. It is also desirable that such low-speed torque be available on demand without the effects of turbocharger lag and other transient effects. If the intake plenum is charged with fresh air plus conventionally-recirculated exhaust gas, several combustion cycles may be necessary to purge the EGR from the system, and this can contribute to poor or delayed torque response. EEVO, while useful for cold starts as described above, can also be used to quickly accelerate the turbocharger for improved torque response. To minimize prior art external EGR delay problems, at least a portion of the EGR can be delivered internally by VVA.
Thus, a still further need is a simple means for providing variable valve actuation in an HCCI engine to match valve events to combustion requirements for various operating conditions.
It is a principal object of the present invention to reduce emissions of oxides of nitrogen and particulates from an HCCI engine.
It is a further object of the present invention to improve cold start characteristics of an HCCI engine.
It is a still further object of the present invention to improve the performance characteristics of an HCCI engine.
It is a still further object of the present invention to reduce the cost and complexity of an HCCI engine.